This invention relates to porous metal-organic frameworks, and to methods of their production and use.
Capturing harmful greenhouse gases, particularly CO2, from industrial effluents could contribute to the minimization of global CO2 content. Porous solids capable of adsorbing CO2 on their surfaces (physisorbing) have been identified as a possible alternative for the existing amine scrubbing process used for industrial CO2 capture. In amine scrubbing, the CO2 is chemically bonded (chemisorbed) to the amine to form a carbamate salt; regeneration of the sorbent requires cleavage of this bond. For capture from flue gas, two major challenges are to capture CO2 at extremely low (0.1 bar) partial pressure and to exclude other major components such as N2 and H2O. Importantly, compared to a chemically bonded entity, the weaker interaction of physisorbed CO2 requires less energy to release the gas for future use or storage. Beyond the greater strength of the CO2 binding interaction, the amine scrubbing process employs an aqueous amine solution so the total energy cost to release CO2 and regenerate the sorbent includes heating the entire solution and water has a very high specific heat capacity. It is typically estimated that, to implement amine scrubbing at a coal-fired power plant, the plant would need to increase energy production by 30-40% to compensate for the CO2 capture. The main benefit of a CO2 physisorbing solid would be to reduce the CO2 liberation energy due both to a weaker interaction of the gas and the absence of a supporting aqueous solution.
Metal organic frameworks (MOFs)/porous coordination polymers (PCPs) are a class of network solids composed of organic spacers linking metal ions or metal ion clusters. They possess ordered (crystalline) structures permeated by pores able to adsorb gas molecules. The regularity of these materials makes them amenable to structural characterization by X-ray diffraction techniques. Detailed knowledge of pore size, shape and surface chemistry can provide greater insights to interpretation of adsorption isotherms. In principle, researchers are able to establish structure activity relationships and better design next generation materials.
Water stability has been shown to be a weakness for many MOFs as even low amounts of atmospheric moisture can compromise the order and the porosity. Some very stable Zr carboxylate MOFs have been reported recently (MIL-140 series, UiO-66 series) but merging high capacity with high stability is still a challenge. Also, with regards to implementation of a capture material, it is difficult to generalize a set of ideal characteristics of the sorbent (pore sizes, heat of adsorption for CO2, shape of isotherm). This is because the characteristics of the ideal sorbent would be specific to the capture system into which it is implemented, broadly pressure-swing, temperature-swing, or vacuum-swing technology. Moreover, the conditions for regeneration would best be dictated by the availability of low cost energy at the specific capture site and this factor would influence selection of the sorbent.